1. Field of the Invention
The invention relates to a process for producing olefin polymer composites having improved melt strength. The process is a solution process whereby an organoclay is incorporated into an olefin polymer base resin. Utilizing the process of the invention, it is possible to produce olefin polymer composites with improved melt strengths using low levels of organoclays and without the use of compatibilizing agents.
2. Description of the Prior Art
Olefin polymers are widely used for the production of film and sheet goods, fibers, molded goods, etc., utilizing a variety of known processes, such as extrusion and coextrusion, blow molding, injection molding, thermoforming and the like.
In extrusion processes, such as those used for the production of films, fibers and molded goods, the olefin polymer resins must have sufficiently low melt viscosity under the high shear conditions encountered in the extruder in order to have acceptable processability and achieve the high throughputs necessary for commercial operation. On the other hand, the resins must have sufficient melt strength after extrusion to prevent sagging/distortion of the extrudate before it is cooled below the resin melt point. High melt strength resins are therefore highly advantageous in these operations. They are particularly advantageous for the production of large thermoformed and blow molded articles. For example, blow molding resins suitable for production of small shampoo bottles may not have sufficient melt strength for the production of one-gallon milk jugs where the parison is substantially larger and heavier. High melt strength olefin polymer resins are also highly desirable for extrusion coating and foamed and sheet extrusion processes.
Whereas some types of olefin polymers, such as low density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) copolymers, are generally regarded to have sufficient melt strength for most of the aforementioned applications, other olefin polymer resins, most notably polypropylene (PP), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins, are deficient in this regard. In fact, these latter types of resins are frequently subjected to post-reactor treatments, such as oxidation treatments during finishing, in order to raise the melt strength of the resins to acceptable levels. While such treatments are capable of increasing the melt strength of certain of these resins, it is often at the expense of other physical properties, e.g., environmental stress crack resistance.
It would be highly advantageous if the melt strength of polyolefin resins could be improved without the use of such post-reactor oxidation treatments. The goal of the present invention is to provide polyolefin composites which exhibit increased melt strength using a straightforward solution technique. It is a further objective to accomplish this using low levels of ammonium-modified clays and without the use of costly compatibilizing agents.
The use of organically modified clays, sometimes referred to as intercalates or organoclays, produced by a cation exchange reaction between the clay and an alkylammonium salt, is known in the prior art. Alkylammonium cations exchanged into and between the clay platelets increase the interlayer spacing between adjacent platelets and render the hydrophilic clays organophilic and thus more easily dispersed in polyolefin resins. Compared to conventional filled polyolefin compositions, polyolefins filled with the intercalated organoclays have improved physical properties at similar loading levels.
Compatibilizing agents are commonly employed to facilitate incorporation of modified clays into non-polar polymers, such as polyolefins, during melt compounding. Compatibilizers are generally maleic anhydride grafted polymers employed at a weight ratio of 3:1 (compatibilizer:organoclay). While polyolefin, composites have been prepared using ratios of compatibilizer to organoclay as low as 1:1, the lower ratios are generally considered to be less desirable and, therefore, are typically avoided. For example, Hasegawa, et al., in an article entitled xe2x80x9cPreparation and Mechanical Properties of Polypropylene-Clay Hybrids Using a Maleic Anhydride Modified Polypropylene Oligomer,xe2x80x9d JAPS 67, 87 (1998), observe that as the ratio of compatibilizer to organically modified clay is increased, the particles of the silicate layers become smaller and are dispersed more uniformly so that the reinforcement effect of the clays is increased.
Lim, et al., in their article xe2x80x9cPhase Morphology and Rheological Behavior of Polymer/Layered Silicate Nanocomposites,xe2x80x9d Rheol Acta 4: 220-229 (2001), report the fabrication of nanocomposites with organophilic modified clays and polyethylene base resins grafted with 0.8 weight percent maleic anyhydride. The nanocomposites are formed by melt-mixing.
Nitrile copolymer and HDPE nanocomposites containing modified silicate fillers are disclosed by Jeon, et al., in xe2x80x9cMorphology of Polymer/Silicate Nanocomposites,xe2x80x9d Polymer Bulletin 41: 107-113 (1998). The HDPE polymer composites contain 20% dodecylamine-modified montmorillonite clay and are obtained by dissolving the HDPE and the modified clay in a co-solvent of xylene and benzonitrile and precipitating in tetrahydrofuran.
In copending application Ser. Nos. 09/947,836 and 10/027,742 propylene polymer composites and ethylene polymer composites having improved melt strength are obtained by incorporating, by melt blending, specific modified organoclays and compatibilizing agents at low compatibilizer levels and at low ratios of compatibilizer to modified clay. Certain ethylene-vinyl acetate copolymer (EVA) composites are obtained without the use of compatibilizing agents.
We have now discovered a solution process whereby it is possible to significantly improve the melt strength and other physical characteristics of olefin polymers by intimately incorporating 0.5 to 10 weight percent of an organically modified clay therein. The process comprises contacting an olefin polymer base resin, said olefin polymer base resin dissolved in an aromatic solvent and selected from the group consisting of ethylene homopolymers, propylene homopolymers, ethylene-propylene copolymers, ethylene-C4-8 xcex1-olefin copolymers, propylene-C4-8 xcex1-olefin copolymers, ethylene-vinyl C2-4 carboxylate copolymers and ethylene-C1-4 alkyl methacrylate copolymers, and an organically modified clay, said organically modified clay dissolved in an aromatic solvent and consisting of a smectite clay that has been ion-exchanged and intercalated with a quaternary ammonium ion of the formula
(R)(R1)(R2)(R3)N+
where R represents a C18 alkyl substituent or mixture of alkyl substituents wherein C18 alkyl moieties constitute 50 percent or more of the mixture and R1, R2, and R3 are independently selected from the group consisting of R, H and C1-22 hydrocarbyl; and evaporating said aromatic solvent to obtain an olefin polymer composite. The resulting olefin polymer composites preferably have from 1 to 7 weight percent, based on the total weight of the composite, of the organically modified clay intimately dispersed therein. Organically modified clays are preferably montmorillonite clays ion exchanged and intercalated with quaternary ammonium ions wherein the R group is comprised of at least 60 percent C18 alkyl groups. Most preferably, the montmorillonite clay is modified with dimethyl dihydrogenated tallow ammonium ion.
Xylene and toluene are highly useful aromatic solvents. In one embodiment of the invention contacting the olefin polymer and organically modified clay is carried out at a temperature of 40xc2x0 C. up to the boiling point of the solvent. Another aspect of the invention utilizes agitation during the contacting step.
Dissolution of the olefin polymer and organically modified clay may be accomplished in the same operation or as separate operations. High shear mixing may be advantageously utilized for dissolution of the organically modified clay and it may also be advantageous to utilize a polar activator compound with the organically modified clay. Recovery of the composite from the solution may be facilitated by precipitating the olefin polymer composite by the addition of a polar compound, such as acetone or methanol. In this way the bulk of the solvent can be removed by filtration or decantation prior to evaporation.
In accordance with the present invention, a process is provided whereby olefin polymer compositions referred to herein as composites, having increased melt strength are produced. Moreover, the process makes it possible to achieve enhanced melt strength and improve other physical properties utilizing low levels of organoclay and without a compatibilizing agent.
The increased melt strength or melt elasticity of the composites of the invention is evidenced by an increase or upturn in their dynamic rheological data at low frequencies. By increase or upturn is meant that the dynamic complex viscosity of the composites increases with decreasing frequencies at frequencies of less than about 1.0 rad/sec compared to the olefin polymer base resins which generally exhibit limiting constant values at frequencies of about  less than 0.1 rad/sec. This complex viscosity increase or upturn, which is measured in terms of xcex94Go*, is an indication that the composites of the invention have superior melt strength compared to the base resins. Enhanced melt strength is also indicated by high complex viscosities (xcex7*) at low frequencies. The relative increase in complex viscosity of the composite as compared to the base resin is expressed by the ratio of complex viscosity of the composite to the base resin at a frequency of 0.1 radians/second.
xcex94Go* is the difference, i.e., increase, in Go* obtained for the composite material over the Go* value observed for the olefin polymer base resin. It can be expressed by the equation:
xcex94Go*=(Go*)compositexe2x88x92(Go*)base resin
xcex94Go* is expressed in dynes/cm2. In general, the higher the xcex94Go* value, the greater the improvement in melt strength of the composite over the base resin. The procedure used to measure the rheological properties of the base resins and composites and determine the viscosity upturn is detailed in the examples.
As will be recognized by those skilled in the art, specific xcex94Go* values referred to herein are provided only to demonstrate the viscosity upturn, i.e., melt strength increase, for the composites of the invention and are not intended to be limiting since they are generated under a specific set of conditions. Rheological data generated using different conditions, e.g., temperature, percent strain, plate configuration, etc., could result in xcex94Go* and complex viscosity ratio values which are higher or lower than those illustrated hereafter.
Olefin polymer resins utilized for the process of the invention include homopolymers and copolymers of ethylene and propylene and mixtures thereof More particularly, base resins utilized for the invention are ethylene homopolymers, propylene homopolymers, ethylene-propylene copolymers, ethylene-C4-8 xcex1-olefin copolymers, propylene-C4-8 xcex1-olefin copolymers, ethylene-vinyl C2-4 carboxylate copolymers and ethylene-C1-4 alkyl (meth)acrylate copolymers.
Representative ethylene homopolymers and ethylene-C4-8 xcex1-olefin copolymers include LDPE, LLDPE, HDPE, very low density polyethylene (VLDPE) and plastomers, medium density polyethylene (MDPE) and very high density or high and ultra high molecular weight polyethylene resins produced using known procedures, such as Ziegler, Phillips or metallocene polymerization processes.
EVA copolymers are representative of ethylene-vinyl carboxylate polymers which can be used and ethylene-ethyl acrylate (EEA) and ethylene-n-butyl acrylate (ENBA) copolymers are illustrative ethylene-alkyl acrylate copolymers.
Comonomer contents of the ethylene-C3-8 xcex1-olefin copolymers will typically range from 0.1 to 45 weight percent and, more preferably, from 0.5 to 40 weight percent. Vinyl acetate contents of EVA copolymers will typically range from 2 and 48 weight percent and, more preferably, from 4 to 45 weight percent. Acrylate monomer contents for the EEA and ENBA copolymers are typically from 1 to 48 weight percent and, more preferably, form 2.5 to 45 weight percent.
Propylene homopolymer, i.e., polypropylene, with varying degrees of isotacticity is also advantageously employed for the invention as are copolymers of propylene and ethylene wherein the comonomers are present in widely varying amounts. Such ethylene-propylene copolymers can include random, block and impact copolymers.
A particularly useful class of ethylene-propylene copolymers wherein propylene is the major comonomer typically containing 55 to 99.5 weight percent propylene and 0.5 to 45 weight percent ethylene. Even more preferred ethylene-propylene copolymers comprise 65 to 99.5 percent propylene and 0.5 to 35 percent ethylene. These weight percentages are for the overall propylene polymer compositions, so that if the composition is comprised of two or more different polymer components such as will be described in more detail to follow, the monomer contents of the individual polymer components comprising the blend may be outside the specified ranges.
In another highly useful embodiment of the process, propylene-ethylene copolymers comprised of two phasesxe2x80x94a continuous phase of highly isotactic polypropylene homopolymer and a dispersed phase of rubber-like propylene-ethylene copolymer, are employed. Depending on the relative proportion of the continuous and disperse phases, these compositions are classified as either impact copolymers or thermoplastic polyolefins (TPOs)xe2x80x94the latter having a significantly higher rubber/elastomer content. Ethylene contents of these polymers will generally range from about 8 weight percent up to about 30 weight percent. While these types of copolymers can be produced by melt compounding individual polymer components, existing multi-reactor technology makes it possible to directly produce these products. This is conveniently accomplished, for example, by polymerizing propylene in a first reactor and discharging the polypropylene homopolymer from the first reactor into a secondary reactor where propylene and ethylene are copolymerized in the presence of the polypropylene. Gas-phase polymerizations of this type are described in the article by Ross, et al., xe2x80x9cAn Improved Gas-Phase Polypropylene Process.xe2x80x9d Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 149-154 which is incorporated herein by reference.
In still another highly useful embodiment, the process of the invention is utilized with HDPE and LLDPE resins to increase their melt strength, Especially useful HDPEs have densities in the range 0.941 g/cm3 to 0.970 g/cm3. Ethylene homopolymers having densities of 0.958 to 0.970 g/cm3 and copolymers of ethylene and butene-1, hexene-1 or octene-1 having densities of 0.941 to 0.959 g/cm3 are especially preferred. Especially useful LLDPE resins have densities from 0.912 to 0.940 g/cm3 and, more preferably, from 0.915 to 0.935 g/cm3 and contain one or more comonomers selected from the group consisting of butene-1, hexene-1 and octene-1. Densities reported herein are determined in accordance with ASTM D 1505.
The melt strength of resins produced using metallocene or single-site catalysts, which typically have low melt strength due to their narrow molecular weight distributions, may also be improved. An example of such ethylene polymer resins is the so-called plastomers and elastomers which have densities between 0.857 and 0.915 g/cm3.
Organoclays employed for the process of the invention are smectite-type clays modified by reacting the clay with a quaternary ammonium ion, i.e., incorporating organic cations between the layers of the clay. The resulting cation exchanged clays exhibit organophilic character which facilitates dispersion in the olefin polymer resins.
Smectite clays are well known in the art as are the methods for modification with quaternary ammonium compounds. Representative natural smectite clays which can be employed include montmorillonite, bentonite, hectorite, saponite, beidellite and the like. Synthesized smectites, such as those synthesized by reacting magnesium silicates and alkali may also be employed. Montmorillonite clays and, more particularly, montmorillonite clays having exchange capacities of at least 50 milliequivalents per 100 grams of clay are preferred.
For the process of this invention, clays of the above types and particularly montmorillonite clays, are reacted with a quaternary ammonium compound having one or more C18 alkyl substituents. More specifically, the quaternary ammonium ion corresponds to the formula:
(R)(R1)(R2)(R3)N+
where R represents a C18 alkyl substituent or mixture of alkyl substituents wherein the C18 alkyl moieties constitute 50% or more of the mixture. R1, R2 and R3 are independently selected from the group consisting of R, H or a C1-22 hydrocarbon group. Mixed alkyl substituents are typically obtained utilizing amines derived from natural sources such as beef tallow or mutton tallow. R moieties obtained from such natural sources correspond to the constituent fatty acids present therein which are typically mixtures of aliphatic radicals comprised of predominantly C14-18 carbon atoms. The carbon number range and distribution within the carbon number range, i.e., percentage of each component, can vary depending on factors such as the tallow source, treatment and age of the tallow. Typical constituent fatty acid values have, however, been generated and are as follows:
Thus, when hydrogenated and even allowing for variations in tallow composition referred to above, it is expected that the C18 alkyl substituents will comprise at least 60% by weight and, more typically, at least 65% by weight of the R groups. More specifically, it is anticipated R would be comprised of approximately 5% C14, 30% C16, and 65% C18 alkykl groups.
Montmorillonite clays modified with dimethyl dihydrogenated tallow ammonium ion (abbreviated 2M2HT) and represented by the formula: 
where HT represents the hydrogenated tallow group are particularly effective for the invention. Montmorillonite clays modified with dimethyl dihydrogenated tallow ammonium chloride at modifier concentrations of 50 to 150 meq/100 g can advantageously be used. Clays with modifier concentrations of 90 to 130 meq/100 g are particularly useful. Modified clays of this type are available from commercial sources at different modifier concentrations. For example, Southern Clay Products, Inc., offers several montmorillonite clay products modified to different levels with dimethyl dihydrogenated tallow ammonium chloride, including Claytone(copyright) HY, Claytone AF, Cloisite(copyright) 6A with a modifier concentration of 140 meq/100 g, Cloisite(copyright) 15A with a modifier concentration of 125 meq/100 g and Cloisite(copyright) 20A with a modifier concentration of 95 meq/100 g.
The process utilized to prepare the composites of the invention having improved melt strength involves dissolving the olefin polymer base resin and organically modified clay in an aromatic solvent and then recovering the composite by removing the solvent. One or more optional steps, which will be described in more detail to follow, may also be utilized and are advantageously employed with certain olefin polymers.
Aromatic solvents used for the process include benzene; alkyl-substituted benzenes such as toluene and xylene; alkylidene-substituted benzenes such as styrene, halogentated benzenes such as dichloro- or trichlorobenzene; and the like. Toluene and xylene are especially useful. In most instances, the amount of aromatic solvent employed will be such that the combined weight percent of olefin polymer and organoclay in the aromatic solvent is 40 percent or less. More typically, the total concentration of the olefin polymer and organically modified clay in the aromatic solvent is from 1 to 25 weight percent and, most preferably, from 5 to 20 weight percent.
The weight ratio of olefin polymer to organoclay will be varied as necessary to achieve the desired amount of organoclay in the resulting composite. Composites obtained by the process of the invention will have 0.5 to 10 weight percent organically modified clay intimately dispersed therein. More preferably, the organoclay constitutes from 1 to 7 weight percent of the total weight of the composite.
Dissolution of the olefin polymer and organically modified clay in the aromatic solvent is accomplished by contacting the mixture until the particles of the base polymer starting resin are no longer visible. The resulting solution may be clear but, more typically, is translucent. While the clay may, strictly speaking, not be soluble in the aromatic solvent, the organically modified platelets are sufficiently dispersed in the solvent so that a xe2x80x9csolutionxe2x80x9d is produced at the concentration limits specified, above. The organoclay and olefin polymer may both be added to the aromatic solvent and dissolved or separate solutions of the organoclay and olefin polymer may be prepared using the same or different aromatic solvents and these solutions subsequently combined. Also, dissolution of the organoclay and olefin polymer may be done in a step-wise manner. For example, the organically modified clay may first be added to the aromatic solvent and, after dissolution of the organoclay, the olefin polymer may be added and the dissolution operation continued until the olefin polymer is dissolved. This approach is particularly useful where it is desirable to utilize different conditions for dissolution of the organoclay and olefin polymer or where a polar activator is employed with the organoclay.
While dissolution can be effected at ambient temperatures, solution of the olefin polymer and organoclay is generally accomplished by heating these components, individually or in combination, in the aromatic solvent until solution is achieved. The time required to achieve dissolution can be as little as 10 minutes up to several hours depending on the temperature used, the aromatic solvent employed, the particular components being dissolved, the type and degree of mixing used, and other variables. When heat is used for dissolution, the temperature can range from just above ambient up to the boiling point of the solvent; however, most typically the temperature will be from about 40xc2x0 C. up to the boiling point of the aromatic solvent. Heating under reflux is particularly advantageous. Once solution has been achieved, additional heating within reasonable limits is not detrimental. Dissolution and particularly dissolution with the application of heat can be advantageously carried out under an inert atmosphere of nitrogen or the like to minimize degradation of the olefin polymer.
Some form of mixing may also be advantageously employed during all or part of the dissolution operation. Mixing may be accomplished by tumbling on a roll mill or by mechanical agitation or the like. In one highly useful embodiment of the invention high shear agitation is employed for dissolution of the organically modified clay, either in the presence or absence of the olefin polymer. High shear mixing is particularly useful when the dissolution operation is conducted in a stepwise manner, first dissolving the organoclay and then adding the olefin polymer and completing the solution operation, either with conventional, i.e., low shear, agitation or high shear agitation.
For the purpose of the invention, agitator speeds of 1000 rpm or below are considered to be low shear or conventional mixing. High shear mixing utilizes stirrer speeds from about 1000 rpm up to about 100,000 rpm. In a preferred embodiment of the process which utilizes high shear mixing, agitator speeds from 4000 rpm to 20,000 rpm are employed.
As previously indicated, polar activators can be employed for dissolution, i.e., solvation of the organically modified clay. While the use of polar activators is not necessary for the process, it may be advantageous in some instances. Polar activators are known for this purpose and are defined as low molecular weight compounds of a polar nature. Most commonly employed polar activators are propylene carbonate, acetone, methanol and ethanol. When alcohols are employed they typically contain at least 5 percent water for maximum effectiveness. Neoalkoxy titanates are also sometimes used as activators. Illustrative activators of this type, also referred to as coupling agents, are neopentyl(diallyl)oxy, tri(dioctyl)pyro-phosphato titanate and neopentyl(diallyl)oxy, tri(N-ethylenediamino)ethyl titanate.
When polar activators are employed, they are necessarily utilized at concentrations which will not interfere with dissolution of the olefin polymer. Since, in one embodiment of the process, some of these same polar compounds are used to precipitate the composite from the aromatic solvent, the concentration of the polar activator used for solvation of the organoclay cannot be such that it will adversely affect the solubility limits of the olefin polymer and prevent it from completely going into solution with the organoclay, i.e., the ability to obtain a solution of the organically modified clay and olefin polymer. In general, the amount of polar activator used for solvation of the organoclay will not exceed 10 weight percent of the total solvent and, more preferably, will be from 0.1 to 3 weight percent.
After both the olefin polymer and organically modified clay are dissolved in the aromatic solvent, the composite is recovered by evaporating the aromatic solvent. This is conveniently accomplished by passing air or, more preferably, an inert gas such as nitrogen over and/or through the solution or pulling a vacuum on the solution. Removal of the aromatic solvent can be facilitated by heating the solution.
In an alternative embodiment, a sufficient volume of a polar compound is added to the solution to cause the composite to precipitate from the solution. The precipitated composite is then isolated by filtration or decanting and any solvent remaining with the composite evaporated. This is accomplished by air or vacuum drying or by passing air or an inert gas over or through the composite. Polar liquids typically added to the solution to precipitate the composite include acetone, methanol, ethanol and tetrahydrofuran.
In addition to having enhanced melt strength, composites produced in accordance with the process of the invention can exhibit other improved physical properties to the base resin. For example, propylene polymer composites can have improved stiffness and heat distortion and HDPE composites typically exhibit improved stiffness and environmental stress crack resistance.
Composites produced in accordance with the process of the invention may be advantageously used for a variety of applications including thermoforming, blow molding, extrusion coating and foamed and sheet extrusion. By using the composites it is possible to eliminate or reduce problems heretofore associated with certain olefin polymer base resins. For example, the upturn in complex viscosity and corresponding increase in melt strength obtained with the composites of the invention make it possible to increase parison hang times. This is highly advantageous when blow molding larger parts which require larger parisons. Also, in thick (0.25-1.5 inch) sheet extrusion processes where resins have a tendency to sag as they exit the die and before they contact the chill roll, use of the composite materials of the invention can eliminate or at least significantly reduce sagging and any operational problems associated therewith. Similar advantages can be realized when the composites of the invention are utilized in thermoforming and extrusion coating processes.
The following detailed examples illustrate the practice of the invention in its most preferred form, thereby enabling a person of ordinary skill in the art to practice the invention. The principles of this invention, its operating parameters and other obvious modifications thereof, will be understood in view of the following detailed procedure. All parts and percentages in the examples are on a weight basis unless otherwise indicated.
Rheological properties were determined using a Rheometrics ARES rheometer. Rheological data were generated by measuring dynamic rheology in the frequency sweep mode to obtain complex viscosities (xcex7*), storage modulus (Gxe2x80x2) and loss modulus (Gxe2x80x3) for frequencies ranging from 0.0251 to 398 rad/sec for each composition. The rheometer was operated at 210xc2x0 C. with the propylene homopolymer and copolymer resins and at 150xc2x0 C. with the polyethylene resins in parallel plate mode (plate diameter 50 mm) in a nitrogen environment (in order to minimize sample oxidation/degradation). Unless otherwise noted, the gap in the parallel plate geometry was 1.2-1.4 mm and the strain amplitude was 10 percent for the polyethylene and propylene copolymer resins and 20 percent for the polypropylene homopolymer. Rheological properties were determined using standard test procedure ASTM D 4440-84.
The measure of viscosity increase or upturn, xcex94Go*, of each composite was determined from the rheological data using a modified Casson plot of G* vs xcfx89 wherein G* is the absolute value of dynamic modulus and xcfx89 is the frequency. The five lowest frequency points for each data set were used to obtain an intercept value Go* at xcfx89=0 by the linear extrapolation method. xcex94Go* is the difference in Go* between the composite and the base resin. It is a measure of the increased complex viscosity of the composite materials of the invention. Go* and xcex94Go* values provided for the examples are reported in dynes/cm2.